Someone posted over at THR or m4carbine.net a letter from the ATF about removing a < 16" upper from a registered SBR lower... the reply from the ATF was that by removing the short upper and replacing the upper with a >= 16" upper, the rifle was no longer under purview of the NFA. The letter also stated that if the change was permanent, a letter should be written to the NFA branch requesting the lower be removed from the registry.